The present invention relates generally to magnetic field sensing devices and specifically to magnetic field sensing devices capable of sensing magnetic fields along two mutually perpendicular axis. Such two axis or dual axis magnetic field sensors are required in many applications. Electronic compasses, magnetic field probes, and virtual reality are a few examples of applications where two axis magnetic field sensors are useful.
In the past, two axis magnetic field sensors were typically constructed using two single axis magnetic sensors. For simplicity, these will be referred to herein as an x-axis sensor and a y-axis sensor meaning that the two axes are perpendicular. The two single axis sensors could be housed in a single package enclosure and oriented so that their sensitive directions were perpendicular to each other. Alternatively, two single axis individually packaged die could be mounted on a circuit board with the sensitive axis of each die perpendicular to the other die. There are disadvantages to the use of two single axis die. One disadvantage of this approach is that it requires extra assembly effort either at the package level or at the board level. In addition, it is difficult to locate the two single axis die so that they are orthogonal to each other. The best control on the orthogonality of the two single axis parts in high volume manufacture may be on the order of ±2°, which induces the same level error on compass heading or about 0.06% error in a magnetic field measurement.
Earlier magnetoresistor-based magnetic field sensors utilized anisotropic magnetoresistance (AMR) wherein the resistance of a magnetoresistor varied as the cosine squared of the angle between the magnetization direction of the magnetoresistor and the direction of the current flow in the magnetoresistor. An example of a magnetic field sensor using AMR elements in a Wheatstone bridge circuit is described in U.S. Pat. No. 5,247,278, assigned to Honeywell.
More recently, a very different principle has been utilized in magnetic field sensors, wherein the resistance of multilayer film with at least two separated ferromagnetic layers varies as the cosine of the angle between the magnetizations of the two layers and is independent of current direction so long as the current is in the plane of the film. This resistance change caused by a magnetic field is named Giant Magnetoresistance (GMR) and is a much greater change than is found in AMR materials. GMR materials are more difficult to construct but offer much greater sensitivity to magnetic fields when compared to AMR materials. The classes of GMR materials that could be used in magnetic field sensors include multilayer film, spin-valve film or spin dependent tunneling film.
A multilayer GMR film includes a stack of alternated ferromagnetic layers and noble metal layers, such as Co/Cu/Co/Cu . . . or NiFeCo/Co/Cu/Co/NiFeCo/Co/Cu . . . When adjusting thickness of the Cu layer, the two magnetic layers adjacent to the Cu layer could have opposite magnetization directions through exchange coupling at zero applied magnetic field and the resistivity is high. Applying a magnetic field in the direction perpendicular to the magnetization directions, the magnetization of each of the ferromagnetic layers rotates toward the applied field direction, the relative angle between the magnetization in the adjacent layer becomes smaller, and the resistivity decreases.
A spin valve film typically includes two ferromagnetic layers, one space layer and one pinning layer. The pinning layer usually is an antiferromagnetic layer used to pin the magnetic moment of the immediate adjacent magnetic layer. The other ferromagnetic layer, separated by the space layer, is free to rotate its magnetic moment. When a magnetic field is applied, the magnetic moment of the free layer will rotate toward the field and the relative angle between the magnetization of the pinning layer and the magnetization of the free layer will change causing a resistance change.
A magnetic field sensor sensitive to very small applied magnetic fields can be constructed utilizing a spin dependent tunneling principle. A spin dependent tunneling magnetoresistor includes first and second ferromagnetic layers separated by a very thin non-magnetic insulating film. Current is conducted through the insulating layer based on a quantum electrodynamic effect or “tunneling” current. For a given thickness of the insulating layer, the amount of tunneling current is largely a function of the directions of magnetization in the ferromagnetic layers located on either side of the insulating layer. These directions of magnetization are varied by the magnetic field to be sensed and the resulting change in resistance is detected. Thus, a need exists for a GMR based two-axis magnetic field sensor that will provide significantly improved sensitivity to magnetic fields, may be readily fabricated on a single semiconductor die, and utilizes many known semiconductor processes.